The unique spectroscopic and magnetic properties of lanthanide ions are widely used to probe the interactions of metal ions with biological macromolecules and cellular membranes. Investigations are often designed to utilize quantifiable chemical differences among the lanthanides to reveal the role of Cations in enzymatic processes, cellular transport phenomena, and membrane Structural determinations. The proposed research will seek to demonstrate that the variation in lanthanide ionic size is an effective probe of the electrical properties of lipid bilayers and biological membranes. The project will test the hypothesis that the sizes of trivalent lanthanide ions influence their affinity and kinetics of binding to the surface chloroplast thylakoid membranes. To substantiate this hypothesis, the principle of features of the interactions of trivalent lanthanides with the thylakoid membrane surface will be determined using two fluorescent probes, 9-aminoacridine (9-AA) and 2-p-toluidinonaphtbalene-6-sulfonate (TNS). As lanthanide cations are adsorbed onto the thylakoid membrane surface, charge neutralization of surface-exposed negative charges occurs. The extent of charge neutralization subsequently alters the proximity of the 9-AA cation or the TNS anion to the membrane surface, as evidenced by variations in the fluorescence levels of theses ionic probes. Measurement of 9AA or TNS fluorescence intensity as a function of lanthanide concentration will reveal the homogeneity of binding sites, the strength of binding, and the competition for binding sites. Measurements of 9-AA or TNS fluorescence intensity as a function of time will enable the kinetic determinations of the rates of cation binding and removal. Differences in the parameters of lanthanide binding, particularly in relation to lanthanide ionic size, will be assessed to further characterize the structural organization of the thylakoid membrane and the mechanism of membrane charge neutralization. This demonstration of the sensitivity of 9-AA and TNS fluorescence to cation size will provide a novel probe of cation-membrane equilibria for the study of other biological systems.